Electrical device, in particular a switching and control unit for motor vehicles

ABSTRACT

The electrical unit has a printed circuit board ( 10 ) supporting the circuit, which includes a power component ( 11 ) which generates heat. In order to dissipate this heat from the power component, the power component rests on a heat conductive layer ( 13 ) which in turn is applied to the upper side ( 12 ) of the printed circuit board. This heat conductive layer further has a portion of the lid ( 18, 19 ) of the housing resting on it, which serves as a cooling area. Alternatively, the cooling area can be a free-standing cooling element. The heat transfer thus takes place via the heat conductive layer ( 13 ) to the cooling element ( 18, 19 ) so that the latter may be applied and formed independently of type and form of the power component.

BACKGROUND OF THE INVENTION

The invention relates to an electrical device having a printed circuitboard carrying an electronic circuit and at least one power component tobe cooled, particularly a switching or control unit for a motor vehicle.In these known devices, power components which heat up intensely aremounted on cooling elements which are fixed on a printed circuit boardor on a housing on which the printed circuit board is secured. Thesecooling elements consist of special cooling profiles by which the powercomponents are spaced from the printed circuit board or project from it.These cooling elements have the disadvantage that they consist ofprofiles having a complicated structure, and that the process ofassembling the device with the profiles can be automated only with greatdifficulty. Due to the design of the cooling elements and the associatedmethod of mounting the power components, these electrical devices haveto be comparatively large, thus taking up a considerable amount ofinstallation space.

In U.S. Pat. No. 4,811,165, an electrical device is described in whichthe electronic components are arranged on a printed circuit board whichcarries an electronic circuit. This is made from a flexible material andlocated on a plate of good thermal conductivity. This has thedisadvantage that heat from the electronic components must be dissipatedthrough the printed circuit board to the heat conductive plate. Inaddition, the (conventional) method of attaching electronic componentsin a wired structure can be achieved only with considerable effort,since additional insulating provisions vis-à-vis the heat conductiveplate are required when the connecting wires or connection electrodesare routed through soldering apertures of the printed circuit board.

SUMMARY OF THE INVENTION

According to the invention, the electrical device comprises a printedcircuit board carrying an electronic circuit and at least one powercomponent to be cooled; a heat conductive layer applied to the printedcircuit board at least in the vicinity of the at least one powercomponent, each of the power components resting flat with their largestface in contact with the heat conductive layer; and a free-standingmetallic body connected to the heat conductive layer and spaced from thepower components to act as a cooling element for dissipation of heatconducted to the free-standing metallic body through the heat conductivelayer from the power components.

Alternatively, instead of a free-standing metallic body the coolingelement can be a part of a housing accommodating the printed circuitboard. In a preferred embodiment spring means can be provided whichpresses the printed circuit board against an interior part of thehousing to establish a good heat conduction between the heat conductivelayer on the circuit board and the housing.

The heat conductive layer can advantageously be a metal cladding, aconductor track or a laminate of the circuit board. It canadvantageously provide a screening for improvement of electromagneticcompatibility.

In contrast, the electrical device in accordance with the invention hasthe advantage that a particularly flat form of the electrical device ismade possible by the arrangement of the power components. The type andform of the power components has no effect on the form and shape of thecooling elements, so that their type and method of attachment can bedetermined by other factors. The attachment of the power components inaccordance with the invention facilitates good heat dissipation whichallows a larger number of power components to be located on the printedcircuit board. Good heat dissipation further allows a higher ambienttemperature, such as prevails in motor vehicles, for example. Thehousing of the electrical device can further be automatically equippedand soldered in a small number of production steps.

BRIEF DESCRIPTION OF THE DRAWINGS

Two embodiment examples of the invention are explained in more detail inthe description which follows and in the drawings.

FIG. 1 is a longitudinal cross-section cutaway view through a firstembodiment of a control unit according to the invention, and

FIG. 2 is a top view of a printed circuit board of the control unit ofFIG. 1.

FIG. 3 is a top view of a printed circuit board in a second embodimentexample of the control unit according to the invention.

FIG. 4 is a cross-sectional view of the control unit shown in FIG. 3taken along the section lines 4—4 in FIG. 5.

FIG. 5 is a cross-sectional view of the control unit shown in FIG. 3taken along the section lines 5—5 in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1 and 2, the number 10 denotes the printed circuit board of anelectronic control unit which carries a circuit which is not shown inany detail. This circuit includes a power component 11 which heats upduring operation and from which the heat must be dissipated.

The upper side 12 of the printed circuit board 10 has a layer 13 ofmaterial with good thermal conductivity applied to it which extends tothe edge 14 of the printed circuit board. The layer 13 of heatconductive material is preferably metallic and may be an appropriatelydesigned conductor track, for example, or a screening surface for theimprovement of the electromagnetic compatibility (EMC), a laminate, acopper cladding, or similar.

This heat conductive layer 13 has the power component 11 mounted to itby its rear face 15 (largest face), and it is affixed in some suitablemanner, for example by gluing, soldering, or screw fixing. The attachedrear face 15 of the power component 11 is smaller than the base area ofthe heat conductive layer 13.

The connection electrodes 16 of the power component 11 extend initiallyparallel to the printed circuit board 10 without touching the heatconductive layer 13, and they are bent clear of the conductive layer andguided into corresponding soldering apertures 17 of the printed circuitboard. The power component is connected to the circuit by soldering.

The printed circuit board 10 has on its upper side 12 a hood-shapedcover 18, preferably a free-standing metallic body, the peripheralflange 19 of which sits on the circuit board at the edge of the circuitboard 10 and thus also on the heat conductive layer 13. The cover 18 orits flange 19 is connected to the printed circuit board 10 in a suitablemanner, for example by gluing, screw fixing, or soldering.

The cover 18 is constructed, at least in the region of the heatconductive layer 13, from a heat conductive material, preferably ametal, and it serves as a cooling surface for the power component 11.

The heat to be dissipated from the power component 11 is thus passed viathe heat conductive layer 13 directly of to a cooling surface, the cover18. There is thus no elaborate mounting of the power component on or toa specifically form adapted cooling surface. The cooling surface istherefore independent of the form of the power component, which meansthat the form of the power component does not influence either the formor the position of the cooling surface. Furthermore, the power componentand the cooling surface do not have to be in close proximity to eachother.

If the printed circuit board 10—as shown as hatched in FIG. 1—isdesigned as a two-layer circuit board, the underside 20 can also becorrespondingly equipped with power components 11. These then also reston a heat conductive layer 21, via which the dissipated heat is passedto a cooling surface, which is not shown.

To improve the heat transfer between the heat conductive layer 13 or 21and the cooling surface, it is possible to structure the surface of theheat conductive layer appropriately. Using suitable methods, it ispossible to emboss a soldered layer or some other heat conductive layerin lattice structure, by a reflow soldering process, for example.

A large contact area between the heat conductive layer 13 and thecooling surface is of further advantage for improved heat transfer.

Where several power components 11 are arranged adjacent to each other onone side 12 or 20 of the printed circuit board, the metallic layers 13or 21 involved should be insulated from each other, i.e. they should notcontact each other.

If the cooling surface, as shown in the embodiment example, is on thecover of a housing or a freely accessible—not shown—cooling element(e.g. cooling angle), then to good advantage it is electricallyinsulated from the heat conductive layer 13. To this end, the coolingsurface may be provided with an insulating layer, an anodized oxidelayer, varnish or similar, in the area of contact. The relevant coolingelement can then additionally be used for the cooling of several powercomponents.

It is further possible to provide the heat conductive layer 13, at leastin the region of the cooling surface, with an insulating layer.

In contrast to this method, it may for example, be expedient in certainapplications for the improvement of the electromagnetic compatibility,to have an electrically conductive connection between the coolingsurface and the heat conductive layer 13. These are then assembledwithout any insulation.

If, for example, a conductor track of appropriate width or a screeningsurface is used as a heat conductive layer, this can be constructedthicker (increased layer thickness) in order to improve the heatconductivity.

In the wiring area of the printed circuit board, a conductive layer witha thickness of, say, 30 μm is applied. This allows conductive tracks forelectrical contact with the components on the circuit board with widthsbelow 200 μm and with similar distances between the conductive tracks tobe produced.

In the areas in which electrical components are to be connected heatconductively to the conductor tracks, the layer thickness of theconductor track acting as the heat conducting layer 13 should beincreased to thicknesses above 50 μm, preferably to layer thicknesses offrom 100 μm to 200 μm.

To improve the heat conductivity further, the printed circuit board 10can be provided with heat conductive layers on both sides (side 12 orside 20), which are connected to each other by a generally knownthrough-contact method. In this way, parallel heat dissipating paths arecreated.

If the printed circuit board 10 is fitted with SMD components(SMD=surface mounted device) in the area of the heat conductive layer,then the surface of the heat conductive layer 13 and that of thecorresponding conductor track section are preferably arranged in oneplane, for good mounting and contact.

If the cooling surface, as shown in the embodiment example, is on thecover of a housing, this can be designed so that an electromagneticscreening of the covered circuit is effected at the same time(improvement of electromagnetic compatibility, EMC). This cover is thenconstructed as a metallic or metallized box which covers the appropriateprinted circuit board areas. This metallic box can then cover the entirearea of the printed circuit board and rests on the edges of the printedcircuit board, where electrical and heat conductive contact is made.Alternatively, it is possible to design the box so that it covers theprinted circuit board only in that region in which sensitive componentsare arranged, i.e. components which need to be screened with regard tothe electromagnetic compatibility of the circuit. The power componentswhich require cooling can be arranged within or outside the box.

In order to connect this box to the printed circuit board in amechanically stable manner and with good conductive capabilities,connecting tabs can be fitted to its underside, which project throughcorresponding apertures of the printed circuit board or which are guidedpast their edges. After soldering the top side of the printed circuitboard, the box is placed on it, and the connecting tabs are bent overand soldered. These connecting tabs can also be used to fix acorresponding bottom part (also an EMC box).

In the second embodiment example of the electronic control device shownin FIGS. 3 to 5, a connection between the heat conductive layer and thecooling element is made in a particularly advantageous manner which iseasily and securely effected during assembly. The cooling element inthis embodiment example is the housing 40 of the control device.Components which are identical to those in the previously describedembodiment example are designated with the same reference numbers.

The printed circuit board 10 is of rectangular design and carries on itsfront face 30 a plug strip 31 for contacting the electronic circuit. Theupper side 12 of the printed circuit board 10 has the layer 13 ofmaterial with good thermal conductivity applied to it. This layer 13 isarranged on the edge regions 32 to 34 in the form of a copper cladding.The layer thickness of this copper cladding or deposit of copper ispreferably between 300 μm and 400 μm. In the embodiment example shownhere, the heat conductive layer 13 is extended to form a continuoussurface on the three free front faces 35 to 37 of the printed circuitboard. The width of the heat conductive layer 13 (at right angles to theouter edge of the printed circuit board) is dependent on the powercomponents 11 to be cooled or the dimensions of these.

The power components to be cooled (wiring components or SMD=surfacemounted devices) are placed on this heat conductive layer 13 such thatthey are at a certain distance from the front face or from the edge ofthe printed circuit board.

The width of the heat conductive layer 13 is matched to the particularstructural shape of the power components, in sections, so that the areaswhich are in contact are as large as possible in order to achieve goodheat transfer, yet still allowing reliable contacting of the particularpower component.

The housing 40 for the printed circuit board 10 is approximatelyrectangular and is parallelepiped shaped open on one front face 41. Itis therefore composed of one base part 42, one lid part 43, and threeside walls 44, 45 and to 46. On each of the inner sides 47, 48 and 49, aflange 50, 51 and 52 is formed which extends parallel to the base part42. The flanges 50 and 51, 51 and 52, respectively, are continuouslyconnected with each other.

On the base part 42, a wedge 53, 54 is formed at the transition to theside walls 44 and 46, respectively, which extends rising from the openfront face 41 to the opposite side wall 45.

The housing 40 is closed on its front face 41 by a front plate 55. Thishas an aperture 56, through which the plug strip 31 projects. On theinner side 57 of the front plate 55, two wedge shaped spring elements58, 59 are provided. The two spring elements 58, 59 are of the samestructure, each has a short fixing section 60, which attaches to theinner side 57 of the front plate 55 and is fixed there. Projecting fromthis at an approximately right angle is an upper section 61, the lengthof which is slightly shorter than that of the flanges 50 and 52. Theupper section 61 merges into a connecting arc 62, from which a lowersection 63 extends. This projects as far as the vicinity of the fixingsection 60. The upper section 61 and the lower section 63 form a wedge,which tapers, starting from the front plate 55.

In the assembled condition of the control unit, the printed circuitboard 10 is between the flanges 50, 51 and 52 and the base part 42 ofthe housing. The front plate 55 closes the housing 40, with the plugstrip 31 protruding through the aperture 56. The spring element 58 restson the wedge 53, while the second spring element 59 rests on the wedge54. In their spring action and shape, the spring elements 58 and 59 areadapted to the wedges 53 and 54 and their distance to the flanges 50 and52 so that the printed circuit board is pressed with its upper sideagainst the flanges, causing the heat conductive layer 13 and theunderside of the flanges to press against each other. The heat transferfrom the power components 11 can thus take place via the heat conductivelayer 13 to the flanges 50 to 52 and consequently to the housing 40.

During assembly of the electrical device, the printed circuit board 10with the plug strip 31 is inserted into the housing 40 together with thefront plate 55. The printed circuit board 10 is then between the flanges50 and 52 and the base part 42 of the housing. During insertion, thespring elements 58 and 59 rest on the wedges 53 and 54, respectively.After insertion, each of the wedge shaped spring elements exerts anadequate contact pressure by action of the wedges 53 and 54, which isrequired for the heat dissipation. The heat is thus dissipated from thepower components via the heat conductive layer to the housing, withoutany additional cooling body or support frame. Nevertheless, even greaterheat outputs can be transferred.

What is claimed is:
 1. An electrical device comprising: a printedcircuit board (10) carrying an electronic circuit and having a pluralityof electrically conducting conductor tracks and a metal cladding as heatconductive layer (13) for heat conduction, said heat conductive layer(13) also serving as an electrically conducting element and beingarranged in an edge region (32 to 34) of said printed circuit board(10); at least one power component (11) to be cooled, said at least onepower component (11) being electrically connected with at least one ofsaid electrically conducting conductor tracks, being arranged in saidedge region (32 to 34) and resting on said heat conductive layer (13) insaid edge region (32 to 34) so that heat generated in said at least onepower component (11) is conducted away by said heat conductive layer(13); and a cooling element for dissipation of the heat conducted fromsaid at least one power component (11) by said heat conductive layer(13) to said cooling element; wherein said heat conductive layer (13) insaid edge region (32 to 34) has a thickness greater than a thickness ofsaid conductor tracks in another region of said printed circuit boardoutside of said edge region (32 to 34).
 2. The electrical device asdefined in claim 1, wherein said thickness of said heat conductive layer(13) in said edge region (32 to 34) is greater than 70 micrometers andsaid thickness of said conductor tracks is approximately 30 micrometersin said another region of said printed circuit board outside of saidedge region (32 to 34).
 3. The electrical device as defined in claim 1,wherein said at least one power component (11) has a largest surface(15) and contacts with said largest surface (15) said heat conductivelayer (13).
 4. The electrical device as defined in claim 1, wherein saidheat conductive layer (13) provides a screening surface for improvementof electromagnetic compatibility.
 5. The electrical device as defined inclaim 1, wherein said heat conductive layer (13) is a laminate of saidprinted circuit board (10).
 6. The electrical device as defined in claim1, further comprising a housing (40) and wherein said cooling element isat least a part of said housing.
 7. The electrical device as defined inclaim 1, wherein said heat conductive layer (13) and said coolingelement are electrically insulated from each other.
 8. The electricaldevice as defined in claim 1, further comprising an electromagneticallyshielded housing (40) including interiorly extending flanges (50 to 52)and wedge-shaped guiding elements (53,54) and wherein said coolingelement is at least a part of the electromagnetically shielded housing.9. The electrical device as defined in claim 8, wherein said at leastone power component (11) is within said electromagnetically shieldedhousing (40).
 10. The electrical device as defined in claim 8, furthercomprising spring means (58,59) arranged in said housing (40) to urgesaid heat conducting layer (13) and said flanges (50 to 52) of saidhousing (40) into contact with each other.
 11. The electrical device asdefined in claim 8, wherein said housing (40) is a push-on-moduleaccessible from a front side (41) thereof.
 12. The electrical device asdefined in claim 10, wherein said spring means (58,59) are wedge-shapedand arranged on said wedge-shaped guiding elements (53,54) of thehousing (40).
 13. An electrical device comprising: anelectromagnetically shielded housing (40) including interiorly extendingflanges (50 to 52) and wedge-shaped guiding elements (53,54); a printedcircuit board (10) carrying an electronic circuit and having a pluralityof electrically conducting conductor tracks and a heat conductive layer(13) for heat conduction, said heat conductive layer (13) also servingas an electrically conducting element and being arranged in an edgeregion (32 to 34) of said printed circuit board (10) in contact withsaid flanges (50 to 52); at least one power component (11) to be cooled,said at least one power component (11) being electrically connected withat least one of said electrically conducting conductor tracks, beingarranged in said edge region (32 to 34) and resting on said heatconductive layer (13) in said edge region (32 to 34), so that heatgenerated in said at least one power component (11) is conducted to saidheat conductive layer (13); a cooling element for dissipation of theheat conducted from said at least one power component (11) by said heatconductive layer (13) to said cooling element, wherein said coolingelement is at least a part of said housing (40); and spring means(58,59) arranged between said printed circuit board (10) and saidwedge-shaped guiding elements (53,54) of said housing to urge said heatconducting layer (13) and said interiorly extending flanges (50 to 52)of said housing into contact with each other; wherein said heatconductive layer (13) has a thickness greater than 70 micrometers insaid edge region (32 to 34) of said printed circuit board and saidconductor tracks have a thickness that is approximately 30 micrometersin another region of said printed circuit board, said another region ofsaid printed circuit board being outside of said edge region (32 to 34)and not provided with said heat conductive layer (13).
 14. Theelectrical device as defined in claim 1 or 13, consisting of a switchingor control unit for a motor vehicle.